Passengers hit by a third day of train strikes in just five days have been told their rights to compensation because of delays are "surprisingly complicated".
On Friday, RMT union members on South Western Railway (SWR) - which operates in areas including London Waterloo and Reading, Arriva Rail North (Northern), Merseyrail and Greater Anglia walked out for 24 hours following previous action on Monday and Wednesday.
The union is striking in a long-running dispute over the role of guards.
The RMT says scrapping guards and introducing driver-only operated trains risks passenger safety, but the the rail companies maintain such a move is safe and already widely used.
Picket lines have appeared at some railway stations affected by the strike, and passengers are facing delays, cancellations and replacement buses in some parts of the country.
Consumer group Which? explained that for passengers who have faced delays due to strike action, "their rights to compensation... are surprisingly complicated", and that if the train company is not planning to run any services, then compensation cannot be claimed.
Alex Hayman from Which?, said: "Frustrated passengers who face yet more disruption to their rail services will be disappointed to find out that their rights to compensation when delayed as a result of a strike are surprisingly complicated.
"You can only claim compensation during a strike if a train was delayed for long enough to qualify or if it didn't turn up at all, based on the revised timetable.
"If operators aren't planning to run any services whatsoever, then unfortunately you can't claim compensation."
Travellers on SWR will face further disruption on Saturday and Sunday due to engineering work which will affect several routes, with replacement bus services in place in areas including Southampton, Brockenhurst, Woking and Guildford.
Which services are running on Friday?
SWR says it plans to run more than 70% of its normal weekday service of 1,700 trains, although there will be rail replacement buses, arrangements to have tickets accepted on other train companies and most routes will see a reduced service.
Northern said it intends to run around 1,350 services on strike days, more than half its normal timetable, mostly between 7am and 7pm.
Greater Anglia plans to run a normal service, with no alterations.
Merseyrail will run a reduced service, mostly between 7am and 7pm, with a break during the middle of the day.
The RMT has criticised Transport Secretary Chris Grayling for not responding to its call for a summit to try to break the deadlock.
General Secretary of the union, Mick Cash said: "RMT members remain rock-solid in each of the separate disputes across the country over rail safety this morning, on the final day of this phase of action, as we continue to fight to put public safety, security and access before the profits of the train operating companies.
"Earlier this week we wrote to Transport Secretary Chris Grayling calling for summit talks under an independent chair to break the deadlock in these long-running disputes.
"He has refused to respond but managed to take time off during his busy schedule this week to stand up in Parliament to defend to the hilt Britain's rip-off private rail companies.
"His lack of interest in rail staff's concerns for passenger safety is a disgrace.
"It makes no sense at all that we have been able to agree long-term arrangements in Wales and Scotland which secure the guard guarantee and which underpin the basic principles of safety, access and security for the travelling public.
"If that rail safety guarantee is right for Wales and Scotland then it must be right for the rest of the UK."
Earlier this week, a spokesperson for the Department for Transport said: "This is a dispute between a private company and the RMT.
"However, the Transport Secretary recognises the disruption caused to passengers and has met with union leaders on several occasions, including as recently as December, to help bring an end to the strikes.
"He offered guarantees of employment to members who currently fulfil the role of the second person on the train beyond the length of the franchises.
"Nobody is losing their job as a result of driver-controlled operation trains - employees have been guaranteed jobs and salaries for several years."
Passengers who believe they are eligible for compensation can make claims through the train operator's website, or by collecting a form from their station.